Hitherto, a sheet finisher is known which is disposed downstream of an image forming apparatus such as a copier, a printer, or a Multi-Function Peripheral (MFP) and performs finishing, such as punching or stitching, on a printed sheet.
Recently, functions of the sheet finisher are diversified, and the sheet finisher is proposed which has, in addition to the punching and stitching functions, a folding function of folding a part of a sheet, and a saddle-stitching and folding function of stapling the center of a sheet and then folding the sheet at the center (see JP-A 2004-106991, U.S. Pat. No. 6,905,118 B2, etc.)
The sheet finisher having the saddle-stitching and folding function can form (bind a book) a booklet from plural printed sheets.
In the saddle-stitching and folding proposed hitherto, after the center of sheets is stitched with a staple or the like, the stitched part is creased by a pair of rollers called fold rollers and is folded. At this time, a plate-like member called a fold blade is applied to the stitched part of the sheet bundle, and is pushed into a nip of the fold roller pair to crease the sheet bundle.
However, a time in which the folded part of the sheet bundle is pressed by the nip of the fold rollers is short, and since the whole folded part is simultaneously pressed by the nip of the fold rollers, the pressure is dispersed to the whole fold. Thus, the fold formed by the fold rollers becomes the fold to which a sufficient pressure is not applied. Especially, when the number of sheets is large, or when the sheet bundle includes a thick sheet, an incomplete fold is often formed.
In order to deal with this problem, JP-A 2004-106991 or U.S. Pat. No. 6,905,118 B2 discloses a technique in which a roller called a reinforce roller is separately provided, and the fold formed by the fold rollers is reinforced by this reinforce roller.
In the technique disclosed in JP-A 2004-106991 or U.S. Pat. No. 6,905,118 B2, the sheet bundle pushed out from the fold rollers is temporarily stopped on a guide plate, and the reinforce roller is moved along the fold while applying a pressure from above to the fold of the sheet bundle. The fold nipped between the guide plate and the reinforce roller is reinforced by the pressure generated between the guide plate and the reinforce roller.
However, in the technique disclosed in JP-A 2004-106991 or U.S. Pat. No. 6,905,118 B2, since the pressure is applied to the fold between the reinforce roller and the plane guide plate, it is anticipated that the pressing force of the reinforce roller is diffused by the plane guide plate, and the pressure to reinforce the fold is not effectively applied to the fold.
A method is conceivable in which a fold is nipped by the nip of a pair of reinforce rollers, the pair of reinforce rollers are moved along the fold while applying a pressure to the nip, and the fold is reinforced. In this method, since the force of pressing the pair of reinforce rollers to each other can be concentrated on one point of the nip, the high pressure is generated at the nip, and the fold can be more effectively reinforced.
Specifically, one reinforce roller (first roller) of the pair can be made to freely rotate in a state where the position is fixed in the thickness direction of a sheet bundle. Meanwhile, the other reinforce roller (second roller) of the pair can be made to freely rotate similarly to the first roller, and can be made to move in the thickness direction while applying an urging force by an elastic member, such as a spring, in the thickness direction of the sheet bundle.
The pair of reinforce rollers having the structure as stated above contact and press to each other by the urging force of the elastic member in an outside region of the sheet bundle in the width direction. When the reinforce roller pair moves from the outside region of the sheet bundle to the edge (edge of the fold) of the sheet bundle, the second roller gets over the edge of the sheet bundle against the urging force of the elastic member, and then moves while pressing the fold by the urging force, and reinforces the fold.
When the sheet size is middle or less (for example, A4 size or less), a region outside the sheet bundle in the width direction can be ensured to a certain degree. Thus, after the reinforce roller pair starts to move from the standby position and before the pair reaches the edge of the sheet bundle, a moving speed of a certain degree can be obtained and the reinforce roller pair can easily get over the edge of the sheet bundle. However, when the sheet size is the maximum size (for example, A3 size) among the processable sizes, the region outside the sheet bundle in the width direction can be hardly ensured. Thus, there can occur a state in which a sufficient moving speed can not be obtained after the reinforce roller pair starts to move from the standby position and before the pair reaches the edge of the sheet bundle, and the reinforce roller pair can not get over the edge of the sheet bundle. In order to obtain the sufficient moving speed, it is necessary to additionally provide an area for run-up at the outside of the sheet width of the maximum size, which becomes a factor to increase the size of the apparatus.